Flexible white reflective dielectric for electronic circuits

ABSTRACT

This invention is directed to a polymer thick film white reflective flexible dielectric composition comprising urethane resin, thermoplastic phenoxy resin, and white reflective powder. Dielectrics made from the composition can be used in various electronic applications to protect electrical elements and particularly to reflect light in 3D circuits containing LED&#39;s.

This application claims priority to the following US PatentApplications: U.S. application Ser. No. 14/326,836 filed Jul. 9, 2014and U.S. Ser. No. 61/860,293 filed Jul. 31, 2013.

FIELD OF THE INVENTION

This invention is directed to a polymer thick film white reflectiveflexible dielectric composition. Dielectrics made from the compositioncan be used in various electronic applications to protect electricalelements and particularly to provide reflectance to maximize LEDlighting.

BACKGROUND OF THE INVENTION

Dielectrics have long been used to protect electrical elements. Theyhave also been used as isolating layers. Although they have been usedfor years in these types of applications, the use of dielectrics asreflective elements is not common. This is particularly important incircuits containing LED lighting where one wants to maximize theavailable brightness through reflectivity. Additionally, more and morecircuitry is being developed where the base substrate that the LED's areattached to needs to be bent and shaped in a non-planar fashion. Thus,the white reflective dielectric must also be flexible i.e., it mustwithstand a 90 degree bend with no cracking. One of the purposes of thisinvention is to alleviate this issue and produce a flexible reflectiveconstruction in which the light generated from LED's can be maximized.

SUMMARY OF THE INVENTION

This invention relates to a polymer thick film white reflective flexibledielectric composition comprising:

-   -   (a) 15-50 wt % of a first organic medium comprising 10-50 wt %        urethane resin dissolved in 50-90 wt % first organic solvent,        wherein the weight percent of the urethane resin and the first        organic solvent are based on the total weight of the first        organic medium;    -   (b) 15-50 wt % of a second organic medium comprising 10-50 wt %        thermoplastic phenoxy resin dissolved in 50-90 wt % second        organic solvent wherein the weight percent of the thermoplastic        phenoxy resin and the second organic solvent are based on the        total weight of the second organic medium; and    -   (c) 1-70 wt % of a white reflective powder;        wherein the weight percent of the first organic medium, the        second organic medium and the white reflective powder are based        on the total weight of the composition.

The invention is further directed to using the white reflective flexibledielectric to form a protective and/or insulating layer in electricalcircuits.

DETAILED DESCRIPTION OF INVENTION

The invention relates to a polymer thick film white reflective flexibledielectric composition for use in electrical circuits.

The substrate commonly used in the types of circuits considered here isusually polyimide or a polyimide laminate construction such as DuPont™CooLam® (DuPont Co., Wilmington, Del.). DuPont™ CooLam® is used forthermal conductivity purposes when high-brightness LED's are present.Further, DuPont™ CooLam® 3D (DuPont Co., Wilmington, Del.) is used forcircuits which are non-planar.

The polymer thick film white reflective flexible dielectric compositioncan also be used in thermoforming electrical circuits, e.g., capacitiveswitch circuits. The substrate commonly used in polymer thick filmthermoformable capacitive circuits is polycarbonate (PC). PC isgenerally preferred since it can be readily thermoformed. However, PC isvery sensitive to the solvents used in the layers deposited on it. Aninappropriate solvent can and will cause cracking or crazing in the PCsubstrate. In the course of producing a 3-dimensional capacitivecircuit, after the thermoforming step, the final step will often be amolding step in which the finished circuit is formed by injectionmolding using a resin such as polycarbonate. This process is referred toas in-molding and involves higher temperatures. Depending on the resinchosen, these temperatures can typically exceed 250° C. for 10-30 sec.Thus the choice of the resins used in the PTF composition is critical.The combination of the resins used in the instant PTF composition hasbeen shown to survive the in-mold process and produce fully functionalcircuitry whereas most resins typically used in PTF compositions willnot.

The polymer thick film (PTF) white reflective flexible dielectriccomposition is comprised of (i) two organic mediums comprising twopolymer resins dissolved in a first organic solvent and a second organicsolvent, respectively, and (ii) white reflective powder. Additionally,powders and printing aids may be added to improve the composition.Herein weight percent will be written as wt %.

Organic Medium

The first organic medium is comprised of a urethane elastomer resindissolved in a first organic solvent. The urethane resin must helpachieve good adhesion to the underlying substrate. The urethaneelastomer must also provide flexibility for the required bending of thecircuit. It must be compatible with and not adversely affect theperformance of the electrical element.

In one embodiment the urethane resin is 10-50 wt % and the first organicsolvent is 50-90 wt % of the total weight of the first organic medium.In another embodiment the urethane resin is 25-45 wt % and the firstorganic solvent is 55-75 wt % of the total weight of the first organicmedium. In still another embodiment the urethane resin is 15-25 wt % andthe first organic solvent is 75-85 wt % of the total weight of the firstorganic medium. In one embodiment the urethane resin is a urethaneelastomer. In another embodiment urethane resin is a polyester-basedcopolymer.

The second organic medium is composed of a phenoxy resin dissolved in asecond organic solvent that may be the same as the first organicsolvent. Different solvents may also be used. The phenoxy resin addshigh temperature capability to the composition which aids in the use ofthis dielectric as a solder mask if required, and also improves moisturepermeability. That is, it helps impede the progress of moisture throughthe composition. In one embodiment the phenoxy resin is 10-50 wt % andthe second organic solvent is 50-90 wt % of the total weight of thesecond organic medium. In another embodiment the phenoxy resin is 20-35wt % and the second organic solvent is 65-80 wt % of the total weight ofthe second organic medium.

In one embodiment, each medium is 15-50 wt % based on the total weightof the composition. In another embodiment, each medium is 15-40 wt %based on the total weight of the composition. In still anotherembodiment, the first organic medium is 15-25 wt % and the secondorganic medium is 25-45 wt % based on the total weight of thecomposition.

Although the preparation of two separate organic media are preferred, ifthe same solvent is to be used for both media a single organic mediumequivalent to the two organic media described above may be used.

The polymer resin is typically added to the organic solvent bymechanical mixing to form the medium. Solvents suitable for use in thepolymer thick film composition are recognized by one of skill in the artand include acetates and terpenes such as carbitol acetate and alpha- orbeta-terpineol or mixtures thereof with other solvents such as kerosene,butyl carbitol, butyl carbitol acetate, hexylene glycol and high boilingalcohols and alcohol esters. In addition, volatile liquids for promotingrapid hardening after application on the substrate may be included. Inmany embodiments of the present invention, solvents such as glycolethers, ketones, esters and other solvents of like boiling points (inthe range of 180° C. to 250° C.), and mixtures thereof may be used.Various combinations of these and other solvents are formulated toobtain the viscosity and volatility requirements desired. The solventsused must solubilize the resin.

White Reflective Powder

The white reflective powder includes such powders as titanium dioxide,barium titanate, alumina, or mixtures thereof. In one embodiment, theamount of white reflective powder is 1-70% of the total weight of theentire composition. In another embodiment the white reflective powder is20-60 wt % of the total weight of the entire composition and in stillanother embodiment the white reflective powder is 30-55 wt % of thetotal weight of the entire composition. In one embodiment the powder istitanium dioxide. It is preferable to keep the particle size of thereflective powder in the range of 0.3-5 microns so as to avoid anycracking issues.

Additional Powders

Various powders may be added to the PTF dielectric composition toimprove adhesion, modify the rheology and increase the low shearviscosity thereby improving the printability. One such powder is fumedsilica where it has been found to significantly improve the resistanceto moisture penetration.

Application of the PTF White Reflective Flexible Dielectric Composition

The PTF white reflective flexible dielectric composition, also referredto as a “paste”, is typically deposited on a substrate, such aspolyimide or a laminate of polyimide, such as DuPont™ CooLam® 3D, thatis somewhat impermeable to gases and moisture. In other constructions,the white reflective flexible dielectric may be deposited over anexisting silver/dielectric construction.

The deposition of the PTF white reflective flexible dielectriccomposition is performed typically by screen printing, but otherdeposition techniques such as stencil printing, syringe dispensing orcoating techniques can be utilized. In the case of screen-printing, thescreen mesh size controls the thickness of the deposited thick film.

Generally, a thick film composition comprises a functional phase thatimparts appropriate functional properties to the composition. Thefunctional phase comprises functional powders dispersed in an organicmedium that acts as a carrier for the functional phase. Generally, thecomposition is fired to burn out both the polymer and the solvent of theorganic medium and to impart the electrically functional properties.However, in the case of a polymer thick film, the polymer portion of theorganic medium remains as an integral part of the composition afterdrying.

The PTF white reflective flexible dielectric composition is processedfor a time and at a temperature necessary to remove all solvent. Forexample, the deposited thick film is dried by exposure to heat at 130°C. for typically 10-15 min.

Circuit Construction

The substrate used is typically polyimide-based as further processingsteps involve exposure to soldering temperatures. The white reflectiveflexible dielectric is printed and dried as per the conditions describedabove. Several layers can be printed and dried. A subsequent step whichmay include bending of the entire unit is typical in the production of3D circuits with as much as a 90 degree bend required. In oneembodiment, the circuit is used as a solder mask.

EXAMPLE AND COMPARATIVE EXPERIMENT Example 1

The PTF white reflective flexible dielectric composition was prepared inthe following manner. The first organic medium was prepared by mixing20.0 wt % Desmocoll 540 polyurethane (Bayer MaterialScience LLC,Pittsburgh, Pa.) with 80.0 wt % dibasic esters (DuPont Co., Wilmington,Del.) organic solvent. The molecular weight of the resin wasapproximately 40,000. This mixture was heated at 90° C. for 1-2 hours todissolve all the resin. The second organic medium was prepared by adding27.0 wt % PKHH (phenoxy) resin (InChem Corp.) to 73.0 wt % dibasicesters and heating as above. The above weight percent are based on thetotal weight of each of the media, respectively. All following weightpercent are based on the total weight of the PTF white reflectiveflexible dielectric composition. 40.0 wt % titanium dioxide powder(DuPont Co., Wilmington, Del.) was then added as the white reflectivepowder and the entire composition was mixed. The composition was thensubjected to the three-roll-mill for two cycles at 150 psi.

The composition, based on the total weight of the composition, was:

25.0 wt % First Organic Medium 35.0 wt % Second Organic Medium 40.0 wt %Titanium Dioxide Powder

A circuit was then fabricated as follows: On a DuPont™ CooLam® 3Dsubstrate, a blanket print of the white reflective flexible dielectriccomposition prepared as described above was printed with a 200 stainlesssteel screen and dried at 130° C. for 10 min. A second print of thecomposition was then printed and dried. The part was inspected and noevidence of crazing or deformation of the underlying substrate wasfound. The circuit was then subjected to a 90 degree bend and tested forcracking/adhesion. No visible signs of cracking were detected, andadhesion before and after the bend was outstanding (5B on ASTM TapeTest). Reflectivity was measured at 90%.

Comparative Experiment A

A circuit was produced exactly as described in Example 1. The onlydifference was that the white reflective flexible dielectric compositionwas not used. Instead, a standard PTF dielectric, DuPont 5036 (DuPontCo., Wilmington, Del.), was used. Here, cracking and adhesion loss wasobserved after the 90 degree bending. Additionally, reflectivity wasonly 30%.

What is claimed is:
 1. An electrical circuit comprising a substrate anda polymer thick film white reflective flexible dielectric formed from apolymer thick film white reflective flexible dielectric compositionconsisting essentially of: (a) 15-25 wt % of a first organic mediumconsisting of 15-25 wt % urethane in dissolved in 75-85 wt % firstorganic solvent, wherein the weight percent of said urethane resin andsaid first organic solvent are based on the total weight of said firstorganic medium; (b) 25-45 wt % of a second organic medium consisting of20-35 wt % thermoplastic phenoxy resin dissolved in 65-80 wt % secondorganic solvent wherein the weight percent of said thermoplastic phenoxyresin and said second organic solvent are based on the total weight ofsaid second organic medium; and (c) 20-60 wt % of a white reflectivepowder selected from the group consisting of titanium dioxide, bariumtitanate, and mixtures thereof; wherein the weight percent of said firstorganic medium, said second organic medium and said white reflectivepowder are based on the total weight of said composition and whereinsaid electrical circuit is fabricated on a polyimide-based substrate andsaid urethane resin and said thermoplastic phenoxy resin remain as anintegral part of said polymer thick film white reflective flexibledielectric.
 2. The electrical circuit of claim 1, wherein said urethaneresin of said polymer thick film white reflective flexible dielectriccomposition is a urethane elastomer or a polyester-based copolymer. 3.The electrical circuit of claim 2, wherein said urethane resin of saidpolymer thick film white reflective flexible dielectric composition is apolyester-based copolymer.
 4. The electrical circuit of claim 1, whereinsaid electrical circuit is used as a solder mask.
 5. The electricalcircuit of claim 2, wherein said electrical circuit is used as a soldermask.
 6. The electrical circuit of claim 3, wherein said electricalcircuit is used as a solder mask.
 7. The electrical circuit of claim 1,wherein said electrical circuit contains LED lighting.
 8. The electricalcircuit of claim 2, wherein said electrical circuit contains LEDlighting.
 9. The electrical circuit of claim 3, wherein said electricalcircuit contains LED lighting.
 10. The electrical circuit of claim 1,wherein said electrical circuit is a capacitive switch circuit.
 11. Theelectrical circuit of claim 2, wherein said electrical circuit is acapacitive switch circuit.
 12. The electrical circuit of claim 3,wherein said electrical circuit is a capacitive switch circuit.
 13. Theelectrical circuit of claim 10, wherein said electrical circuit has beenthermoformed.
 14. The electrical circuit of claim 11, wherein saidelectrical circuit has been thermoformed.
 15. The electrical circuit ofclaim 12, wherein said electrical circuit has been thermoformed.
 16. Theelectrical circuit of claim 13, wherein said electrical circuit has beensubsequently subjected to an injection molding process.
 17. Theelectrical circuit of claim 14, wherein said electrical circuit has beensubsequently subjected to an injection molding process.
 18. Theelectrical circuit of claim 15, wherein said electrical circuit has beensubsequently subjected to an injection molding process.